Hot melt adhesives are used in a wide variety of applications, such as sealing packaging, labelling articles and product assembly.
Hot melt adhesives are well known in the art and can be formed from polyolefins, for example polyethylene and polypropylene, styrenic copolymers, for example styrene-butadiene-styrene and styrene-ethylene/butylene-styrene, polyamides, polyurethanes, polyesters, polycarbonates, acrylics, ethylene vinyl acetate and compatible mixtures and/or copolymers of two or more thereof.
Notable properties of hot melt adhesives include surface energy, tack, set time, melt flow, melt viscosity, gel content, skin formation and colour stability. It is important that hot melt adhesives maintain these properties during storage, handling and subsequent application. In particular, it is important that the colour, viscosity, and minimal gel and skin formation are maintained during prolonged exposure to heat, which is common in hot melt adhesive applications.
To aid colour retention, additives are usually added to hot melt adhesives. The additives will typically comprise phenolic antioxidants, phosphite antioxidants, or combinations thereof.
To help prevent gel formation it is known to add anti-gel agents to hot melt adhesives, for example polyethylene glycols, polyethylene oxides and ethoxylated linear alcohols.
US 2012/0328805 describes a hot melt adhesive composition that includes a non-functionalized amorphous poly alpha olefin polymer, a second polymer selected from the group including propylene homopolymers, propylene copolymers and combinations thereof, a functionalized wax and a second wax. In addition to these components, the document describes antioxidants which may be present in the composition. These antioxidants are high molecular weight hindered phenols, in particular Irganox® 1010 and Irgafos® 168, multifunctional phenols and phosphites.
US 2012/0329353 describes a hot melt adhesive composition that includes a non-functionalized metallocene catalysed polymer and a second polymer selected from the group including amorphous poly alpha olefins, uni-modal metallocene catalysed polymers, hydrogenated styrenic block copolymers and combinations thereof. In addition to these components, the document describes antioxidants which may be present in the composition. These antioxidants are high molecular weight hindered phenols, in particular Irganox® 1010 and Evernox® 76, multifunctional phenols and phosphites.
WO 2012/159867 describes a wax-containing hot melt adhesive comprising 1 to 40% of a styrenic block copolymer, 3 to 15% of a modified thermoplastic elastomer comprising —COO— groups, and 5 to 25% of a wax. In addition to these components, the document describes antioxidants which may be present in the hot melt adhesive. These antioxidants are high molecular hindered phenol, in particular Irganox® 1010 and Irganox® 1076, sulphur or phosphorous containing phenol and UV absorbers.
US 2013/0060215 describes a hot melt adhesive composition including a semi-crystalline copolymer of propylene and a polyethylene wax. Additionally, the document describes antioxidants which may be present in the composition. These antioxidants are hindered phenols, in particular Irganox® 1010.
US 2012/0165455 describes a hot melt adhesive formulation utilising a blend of polymers which includes a polyolefin polymer and an olefinic block copolymer. The adhesive formulation typically includes a stabiliser or antioxidant selected from high molecular weight phenols, multifunctional phenols, thiodipropionate esters and phosphites.
US 2013/0186566 describes a hot melt adhesive for bonding a film/foil selected from metal film, metal foil, paper film, paper foil, plastic film or plastic foil, wherein the hot melt adhesive comprises 10 to 80% of at least one thermochemically degraded polypropylene copolymer, 5 to 60% of at least one tackifying resin, 0 to 40% of at least one further additive, wherein the hot melt adhesive has a Shore D hardness above 50 measured in accordance with DIN EN ISO 868:2003. The further additive may be a stabiliser selected from hindered phenols and multifunctional phenols such as sulphur- or phosphorous-containing phenols, in particular Irganox® 1010.
U.S. Pat. No. 4,835,200 describes a hot melt adhesive composition containing a block copolymer prepared using a bromide based coupling agent, a tackifying resin, and an effective amount of a stabilisation composition. The stabilisation composition contains a thio compound, a phenolic antioxidant, a tris-(nonylated phenyl) phosphite, and an aliphatic compatible epoxy compound. The thio compound specifically described is 2,4-bis-(n-octylthio)-6-(4-hydroxy-3,5-di-t-butyl anilino)-1,3,5-triazine (Irganox® 565—CAS 991-84-4).
U.S. Pat. No. 5,128,397 describes a hot melt adhesive composition based on ethylene-vinyl acetate copolymer and a hydrocarbon tackifying resin and containing a stabilizer system of a hindered phenol antioxidant, a phosphite costabilizer and an acid scavenger.
U.S. Pat. No. 5,149,741 describes a hot melt construction adhesive which includes an SIS copolymer containing at least about 25% styrene, a compatible tackifying resin, a plasticizing oil and an effective amount of a stabilizer.
However, there are numerous disadvantages associated with the prior art constructions, namely, discolouration, gel formation, skin formation and changes in viscosity, particularly when the hot melt adhesives are exposed to heat for prolonged periods of time.